The present invention relates to temperature measuring devices and, more particularly, to precision thermocouple type thermometers where fast reading, low cost and clinical accuracy is desired.
While the subject invention has broad application its advantages can best be appreciated by considering its application to clinical measurements. Clinical electronic thermometers are presently known and usually employ either thermistors or semiconductor thermal sensors. Although these components along with their peripheral electronics are relatively inexpensive, both types of sensors have disadvantages which limit their use for rapid measurement with high accuracy over a wide temperature range. The performance of both the thermistor and semiconductor thermal sensor is limited to relatively slow direct or indirect measurements, these components having thermal rise times as long as 30 to 60 seconds as a consequence of their significant mass and nonhomogeneous construction. The voltage response with temperature of a thermistor is linear over only a very narrow range. Therefore, accuracy of thermistor thermometers for clinical use has been limited to narrow ranges, typically 96.degree. F. to 109.degree. F. If it is desired to expand the range of such devices, it becomes necessary to provide range switching with each range separately calibrated because the rate of response in different temperature ranges varies markedly. To avoid this problem it is necessary to use external resistor networks to linearize the action of the thermistor. Thus, it is difficult to use this type of construction with integrated circuits.
Both thermistor and semiconductor thermal sensors when used for clinical measurements are not amenable to use in the form of expendable probes both because of their relative expense and the significant differences in response from probe to probe. If the probe were to be changed from patient to patient, the devices would have to be recalibrated each time. The known thermometers of this type, therefore, use permanent probes with disposable envelopes or covers for the probes which covers increase the thermal mass and measurement time even more.
It is well known that thermocouples generate voltage as a function of the metals or alloys incorporated therein, are consistent from thermocouple to thermocouple and are readily interchangeable. Sensitive thermocouples can be produced inexpensively and have very fast thermal response times, usually less than 1 millisecond. Nevertheless, the use of thermocouples in thermometers, particularly clinical thermometers, has been limited as a consequence of costly peripheral electronics and the need for a precise cold junction compensator. For example, one method which has been used is to maintain the cold junction at a fixed known temperature by placing the cold junction within an ice bath so that the temperature is known to be 32.degree. F. However, this method is too cumbersome for practical use. In the alternative, expensive and complicated measuring or temperature stabilizing devices must be used where any finer degree of accuracy is required.